1. Field of the Invention
The field of art to which this invention pertains includes pressurized fluid control systems for multiple speed ratio powershift transmissions. More particularly, the control system is adapted to automatically shift the transmission from a higher speed ratio to a lower speed ratio at a first predetermined vehicle ground speed and thereafter shift back to said higher speed ratio at a second higher predetermined vehicle ground speed. It is the function of the downshift inhibitor circuit to prevent downshifting of the transmission during vehicle full throttle directional reversals.
2. Description of the Prior Art
In front end loader applications, for example, during bucket loading at the pile portion of the work cycle, the vehicle is required to crowd the pile as well as pry or lift with the bucket. The pile is normally approached in second gear from a travel portion of the work cycle, with initial crowding and bucket work being done in second gear. During the second gear operation, in comparison with first gear operation, less torque is available at the driving wheels which has a desirable effect of minimizing wheel spin. However, when additional hydraulic demands are placed on the engine, as a result of prying or bucket lifting operations, insufficient power remains for transmission to the driving wheels for effectively crowding the pile. Therefore, at this time, the operator must normally shift to first gear and, upon loading the bucket and backing out of the pile, he must then manually shift to second gear. In order to relieve the operator from excessive shifting between first and second gears, it is very desirable to have an automatic shift system so that, when the vehicle ground speed drops to a first predetermined speed, the transmission is automatically downshifted from second to first gear which in turn is followed by an automatic upshift when the vehicle ground speed thereafter exceeds a second higher predetermined speed. A transmission shift control system that provides this automatic function is set forth in my co-pending Application Ser. No. 779,445, filed on Mar. 21, 1977, which is also assigned to the assignee of this invention.
For the same vehicle speed, during full throttle operation, it is desirable to perform the directional reversal of the vehicle in second rather than in first gear because of the inherent lower rotational speeds of both the forward and reverse clutch components in second gear operation. Full throttle shifts into and out of first gear are quite harsh, therefore, a principal object of this invention is to provide means fo prohibiting downshifts from second to first gear while the vehicle undergoes full throttle directional reversals. The description of my co-pending application is fully repeated hereinafter to assure full understanding of its structure and function which are considered necessary to fully understand the function and appreciate the benefits of my downshift inhibitor or delay circuit.
Prior art patents wherein shift operation is effected by electrohydraulic means responsive to operating parameters include U.S. Pat. Nos. 3,732,755 to Beig et al; 3,403,587 to DeCastelet; and 3,665,779 to Mori. Furthermore, a downshift inhibitor for a powershift transmission is also shown in U.S. Pat. No. 3,937,107 to Lentz.